Fix boost include error occurring in Ubuntu 16.04.

sunmodel: accept animated transformations (even if animations aren't directly supported within SunModel)

Fixed an incorrect radiance conversion factor in 'coating' and 'roughcoating' reported by Attila Afra.

The models now produce the right falloff compared to Monte Carlo
reference renderings with explicitly instantiated layers

Implementation of the paper 'Accurate computation of single scattering in participating media with refractive boundaries' contributed by Nicolas Holzschuch